A known apparatus for inflating an inflatable vehicle occupant protection device, such as an air bag, includes a pressure vessel for storing a quantity of inflation fluid under pressure. The inflation fluid comprises a combustible mixture of gases. Typically the pressure vessel includes a rupturable burst disk. When the vehicle encounters conditions indicative of a collision of a magnitude requiring inflation of an air bag to help protect a vehicle occupant, an igniter means is actuated. The igniter means ignites the combustible mixture of gases. Ignition of the gases creates hot combustion products and increases the pressure in the pressure vessel to a level sufficient to burst the burst disk. When the burst disk is ruptured, an outlet is created in the pressure vessel for the hot gas effluent. A passageway directs the hot gas effluent from the outlet in the pressure vessel to the air bag.
After the burst disk ruptures, the separated disk falls into the passageway where the hot gas is flowing. Retention of the separated disk in the center of the passageway downstream from the opening would aid in the operation of the apparatus.
The igniter means which ignites the gases commonly includes a projectile which is propelled into and subsequently penetrates the burst disk prior to rupture of the burst disk. The projectile enters the pressure vessel followed by a jet of hot combustion products which then ignites the combustible mixture of gases. The hot combustion products, such as hot particulates, are the result of a combustion process which propels the projectile. The jet of hot combustion products is formed behind the projectile and follows the projectile into the pressure vessel, despite being opposed by the flow of the inflation fluid out of the pressure vessel. To help channel the jet flow efficiently into the pressure vessel, it is desirable to minimize peripheral flow of the jet around the edges of the opening in the burst disk created by the penetration of the projectile.